The invention relates to a method for converting thermal energy into mechanical work, a first and a second means for storing thermal energy being connected alternately into a turbine branch.
DE 43 17 947 C1 discloses a method for converting thermal energy of a gas into mechanical work. In this case, air is compressed quasi-isothermally. The compressed air is then heated isobarically by means of a regenerator. The hot compressed air is then heated further by means of burners in a second step and finally expanded into a gas turbine. Part of the hot compressed air is branched off before it enters the burner and is used for preheating the fuel required for the operation of the burner. The expanded warm waste air emerging from the gas turbine is led via a second regenerator, which is connected alternately into the turbine branch instead of the regenerator. The heating of the compressed air by means of a regenerator and a burner connected downstream is complicated. In the case in which solid fuels are used, special degassing equipment has to be provided here.
DE 44 26 356 A1 discloses an apparatus for drying green stuffs, chips and the like to produce heat and power. In this case, biomass is burned in a combustion chamber. The hot flue gas passes over a heat exchanger. There, it gives up its heat to previously compressed and therefore already heated air. The hot compressed air is expanded into a gas turbine. The warm waste air emerging from the gas turbine is used to dry chips, shavings, green stuffs or the like. However, it can also be supplied to the combustion chamber as preheated combustion air. The apparatus is primarily a drying apparatus. It is not suitable for the efficient production of power. The efficiency relating to the conversion of the thermal energy into mechanical work is not particularly high here.
EP 0 654 591 A1 discloses an apparatus for obtaining electrical energy from fuels. In this case, compressed air is heated by means of a plurality of heat exchangers connected one after another and then led to a turbine. The known apparatus is of complicated construction. It is not particularly efficient.
DE 39 31 582 A1 describes a method for using high-temperature waste heat. In this case, two regenerative heat stores are provided, which are switched alternately from a waste gas to a turbine branch. Similar methods are disclosed by Patent Abstracts of Japan JP 62085136 A and JP 61028726 A. The efficiency of these methods is improved but not optimal.
It is an object of the invention to eliminate the disadvantages of the prior art. In particular, a method is to be specified which permits efficient conversion of thermal energy obtained from the combustion of biomass into mechanical work. It is a further aim to specify a cost-effective apparatus for implementing the method.
This object is achieved by the features of claims 1 and 14. Expedient refinements emerge from the features of claims 2 to 15 and 16 to 22.
According to the invention, a method of converting thermal energy into mechanical work is provided, a first and a second means for storing thermal energy being connected alternately into a turbine branch, with the following steps:
a) compressing an oxidizing gas, its temperature being raised from ambient temperature RT to a first temperature T1 and its pressure being raised to a first pressure P1,
b) cooling the compressed gas down to a second temperature T2,
c) leading the compressed gas through a first means for storing thermal energy, the temperature of the gas being raised to a third temperature T3 in one step,
d) expanding the first pressure P1 in a gas turbine to substantially atmospheric pressure, the gas being cooled down from the third temperature T3 to a fourth temperature T4,
e) feeding the gas to a combustion chamber connected downstream,
f) burning biomass together with the gas, and
g) leading the flue gases through a second means for storing thermal energy.
The oxidizing gas considered is, for example, air, oxygen, air enriched with oxygen and the like. Because the compressed gas is raised to the third temperature in one step, that is to say without the interposition of further heat sources, the method can be managed particularly efficiently. The achievable total efficiency is around 74%.
The cooling in step b can be effected by acting with liquid, preferably water, or by means of a heat exchanger. The thermal energy obtained by the cooling in this step can be coupled out as useful heat. This increases the efficiency of the method further.
In the step c, a partial stream of the gas can be led through a bypass past the means for storing thermal energy. The waste-gas losses of the entire apparatus can thereby be reduced.
After step d, part of the gas is expediently branched off and, in order to couple out thermal energy, is led over a heat exchanger and/or brought into contact with the biomass to be burned in order to dry the latter. According to a further refining feature, dust is separated off from the flue gases formed during the combustion. The separation can be carried out, for example, by means of a cyclone.
It has proven to be particularly advantageous for the flue gas to be cooled down to a temperature of less than 150xc2x0 C., preferably 90 to 110xc2x0 C., in less than 200 ms in step g. This suppresses the formation of a polluting dioxines and furans.
For the temperature of the gas, it is expediently true that:
T2 less than T1 less than T4 less than T3.
The second temperature T2 is preferably less than 150xc2x0 C., in particular less than 100xc2x0 C. By means of the regenerators, gas and flue gas are preferably alternately led through a bulk material with a maximum grain diameter, which is accommodated in the annular chamber between a substantially cylindrical hot grate and a cold grate surrounding the latter, and at least one discharge opening for discharging the bulk material being provided in the bottom of the annular chamber, a predefined quantity of bulk material being discharged while or after the flue gas is led through, so that a compressive stress exerted on the hot and cold grates by the bulk material is reduced. The average grain size of the bulk material is preferably less than 15 mm. The so-called bulk material regenerators operated in accordance with the aforementioned method are particularly efficient and not likely to need repair.
The bulk material discharged can be fed back by means of a transport gas into the annular chamber. In the process, after the bulk material has been separated from the transport gas, a dust fraction contained therein is separated off. The function of the filling as a filter for dedusting the process gas led through is maintained. Furthermore, the transport gas discharged into the environment is unloaded.
In order to manage the process quasi-continuously, a third means for storing thermal energy can be connected into the turbine branch in alternation with the first or second means for storing thermal energy.
According to a further measure of the invention, an apparatus for converting thermal energy into mechanical work is proposed, the following being provided:
a turbine branch comprising:
a compressor for compressing an oxidizing gas taken in,
a means connected downstream for cooling down the compressed gas,
a first bulk material regenerator connected downstream of the cooling means,
a gas turbine connected downstream of the first bulk material regenerator,
and a preheating branch comprising
a combustion chamber connected downstream of the first bulk material regenerator,
a second bulk material regenerator connected downstream of the combustion chamber and
a means for the alternate connection of the second bulk material regenerator into the turbine branch and the connection of the first bulk material regenerator into the preheating branch.
The bulk material regenerators used here are preferably the bulk material regenerators disclosed by DE 42 36 619 C2 or EP 0 908 692 A2. The disclosure content of the aforementioned documents is hereby incorporated. The use of such bulk material regenerators leads to an apparatus which operates particularly efficiently.
The gas turbine, the compressor and a generator are expediently arranged on one shaft. As a result of the direct transfer of the mechanical work from the gas turbine to the compressor and the generator, frictional losses are avoided.
According to a further configuration feature, the combustion chamber can have a flue gas cleaning device connected downstream. Furthermore, a heat exchanger for coupling thermal energy out can be connected in between the gas turbine and the combustion chamber. The aforementioned features contribute further to environmental compatibility and to increasing the efficiency of the apparatus.
According to a particularly preferred configuration feature, a substantially cylindrical hot grate in the bulk material regenerator is surrounded coaxially by a cold grate, and a bulk material with a maximum grain diameter Dmax is accommodated in an annular chamber formed between the hot and the cold grates, the hot and/or cold grates being constructed in such a way that the bulk material can expand radially when heated. Such a bulk material regenerator has a particularly long service life. For details, reference is made to EP 0 908 692 A2. Such bulk material regenerators are distinguished by particularly low energy losses and high heat recovery.
With the apparatus according to the invention, high electrical efficiencies can be achieved even at low electrical outputs, that is to say less than 2 MW. Furthermore, by means of the apparatus according to the invention, heat can be coupled out and supplied to a separate circuit, for example as useful heat for drying purposes.